Brushless DC motors include a rotatable rotor having a constant magnetic field, as well as a stator which can be used to generate a rotating magnetic field. The rotating magnetic field generates a torque that is not constant. In fact, fluctuations are produced in the torque that are a function of the construction of the rotor and the stator, and of their position with respect to each other. In the prior art, it is known to choose various combinations of the number of stator slots and the number of rotor poles depending on the desired motor parameters, such as the output of the motor, cogging torque and redundancy of the motor windings. This means, for example, that a motor having nine pole heads or slots in the stator and sixteen magnetic pole pairs in the rotor has the advantage that it generates very low cogging torque with 144 cogging torque cycles per rotation. Here, the rotor and stator are designed in such a way that the rotor poles have different angular offsets compared to the stator pole heads in order to thus compensate the transition of a rotor magnet from one pole head to the next by another pole head, which, in the same rotor position, lies opposite the center of another magnetic pole.
Such specifications for the structure of the rotor and the stator can produce a winding pattern for the winding of the stator in which for each phase, one slot or even several slots take on windings of the same phase with an opposed winding direction and thus opposed energization. The currents in the opposed winding sections of the same phase located in these slots offset each other and thus make no contribution at all to the build up of torque; the windings simply increase the winding resistance. The winding space in these kinds of motors is consequently not utilized effectively, which means that the motors need to have a comparatively large volume should a certain motor output be required. This results in low efficiency and relatively high costs in the manufacture of such a motor.
The object of the invention is to provide an improved brushless DC motor. The particular aim is to provide a DC motor which is not only compact and low-cost but also shows high efficiency. Moreover, the intention is to provide a DC motor that is light.